Mammalian skeletal muscle fibers have been traditionally classified into specific types by characterization of their myofibrillar ATPase and metabolic histochemistry and more recently by immunohistochemistry with myosin heavy chain (MHC)-specific antibodies. Human skeletal muscle contains three main fiber types: I, IIA and IIX (or IIB) derived from specific MHC isoforms. Mammalian jaw-closing muscle fibers are notably different in comparison to other skeletal muscle, reflecting the specialized nature of their functions. The jaw-closing muscle fibers of the human primate are the most exceptional, expressing developmental and cardiac MHC and myosin light chain (MLC) isoforms not found in other adult skeletal muscle, as well as the typical adult skeletal isoforms. A full description of myosin expression in human masseter is presently incomplete, and a fiber type classification scheme, incorporating both myosin protein content and contractile function, is lacking. Using human masseter muscle biopsies, the first specific aim of this study is to develop a fiber type classification scheme for jaw-closing muscles based on MHC and MLC identification as determined by tissue staining for: myofibrillar ATPase and metabolic histochemistry, immunohistochemistry with MHC-specific antibodies and in situ hybridization with probes developed from different MHC gene transcripts. In whole muscle biopsies and in single fibers, MHC content will be confirmed by glycerol-SDS- PAGE, and MLC content by 2-D electrophoresis. In addition, electron microscopy will be used to evaluate the relative heterogeneous distribution of myosin isoforms within representative fibers. The second aim of this study is to determine the maximum speed of unloaded shortening (Vo), as determined by the slack test, and specific tension (Po) of the individual chemically skinned or freeze-dried fibers representative of each fiber type. Following physiology, the relative percent content of MHC and MLC isoforms in these single fibers will also be correlated to Vo and Po to determine if these physiologic parameters are related to contractile protein expression as is the case in other human skeletal muscle. The long-term objectives of this work are to provide detailed, basic information regarding the exceedingly complex contractile protein expression in human jaw-closing muscle, the classification of specific fiber types distinct or similar to other skeletal muscle, and the main physiological properties of each fiber type. This information may serve as a basis from which dentistry may apply a more thorough understanding of jaw-closing muscle function to common clinical problems such as temporomandibular joint disorders, abnormal craniofacial growth and stability after orthognathic surgical procedures.